A high-energy heat treatment for a workpiece made of steel, such as a high-frequency induction heat treatment, a laser heat treatment, an electron-beam heat treatment and a flame heat treatment, is carried out by directly heating a workpiece, and therefore, has advantages that, for example, relatively high energy efficiency may be able to be achieved as compared with a heat treatment conducted using an atmosphere heating furnace that heats a workpiece through an atmosphere. For example, according to the high-frequency induction heat treatment, a working environment is clean and a relatively small lot of products can be efficiently processed in a short time, as compared with the heat treatment using an atmosphere heating furnace. In high-frequency quenching of steel, generally, by repeating the procedure of heat-treating a test piece while changing a transition of electric power (electric power pattern) with respect to passage of time and checking the quality of the heat-treated test piece, the electric power pattern (heating condition) is set in a trial-and-error manner. In this case, the electric power pattern defined as a heat treatment condition needs to be set each time in accordance with the type (material and shape) of the workpiece. This poses a problem that time and efforts are required for determining the heat treatment condition.
Furthermore, the high-energy heat treatment including a high-frequency induction heat treatment is carried out as partial heating by which a part of a workpiece is heated, with the result that temperature unevenness occurs within the workpiece. Accordingly, when performing a heat treatment such as immersion quenching for heat-treating an entire workpiece, quality unevenness may occur within the workpiece. Particularly, in such a workpiece as having a relatively large thickness, temperature unevenness is more likely to increase, which may lead to a problem that desired quality cannot be achieved in some parts of the workpiece.
In this way, in the case where the entire workpiece is heat-treated, for example, immersion-quenched, by means of a high-energy heat treatment, there occur problems that time and efforts are required for determining the heat treatment condition and also that the heat treatment quality becomes uneven.
In response, there has been a proposed method of suppressing unevenness of the heat treatment quality while reducing the time and efforts for determining the condition by automatically determining the heat treatment condition (for example, see Japanese Patent Laying-Open No. 2006-152430 (PTD 1)). The method disclosed in PTD 1 is implemented by simultaneously measuring the temperature of a region of a workpiece in which the temperature is more likely to rise during high-frequency induction heating and the temperature of a region of the workpiece in which the temperature is less likely to rise during high-frequency induction heating; determining in real time during heating based on the measured temperature data whether the predetermined heat treatment quality is achieved or not in both of these regions; and setting the heat treatment quality of the entire workpiece to a predetermined level. Also, in the method disclosed in PTD 1, it is determined by prediction according to the following formula (A) whether the predetermined heat treatment quality is achieved or not in the entire workpiece.∂C/(∂t)=D∂2C/(∂x2)  (A)
where D: diffusion constant, C: carbon concentration (mass %), t: time (second), x: distance, D=D0exp(−Q/RT)
where D0: entropy term of diffusion constant, Q: activation energy, R: gas constant, and T: absolute temperature (K).